In its most commonly practiced form silver halide photography employs a film in a camera to produce, following photographic processing, a negative image on a transparent film support. A positive image for viewing is produced by exposing a photographic print element containing one or more silver halide emulsion layers coated on a reflective white support through the negative image in the camera film, followed by photographic processing. In a relatively recent variation negative image information is retrieved by scanning and stored in digital form. The digital image information is later used to expose imagewise the emulsion layer or layers of the photographic print element.
Whereas high bromide silver halide emulsions are the overwhelming commercial choice for camera films, high chloride cubic grain emulsions are the overwhelming commercial choice for photographic print elements. It is desired in high chloride emulsions for color paper applications to obtain high photographic speed at the desired curve shape. While it has been common practice to avoid or minimize the incorporation of iodide into high chloride grains employed in color paper, it has been recently observed that silver iodochloride cubical grains can offer exceptional levels of photographic speed where iodide is incorporated in such emulsion gains in a profiled manner. Chen et. al. in U.S. Pat. No. 5,547,827; Chen et. al. in U.S. Pat. No. 5,550,013; Chen et. al. in U.S. Pat. No. 5,605,789; Chen et. al. in U.S. Pat. No. 5,726,005; Edwards et.al. in U.S. Pat. No. 5,728,516; Chen et. al. in U.S. Pat. No. 5,736,310; Budz et.al. in U.S. Pat. No. 5,783,372 and Edwards et.al. in U.S. Pat. No. 5,792,601 disclose highly sensitive silver iodochloride cubical emulsions with low levels of iodide located in the exterior portions of the grains. The interior portions of such grains can be prepared by employing any convenient high chloride cubical grain precipitation procedure. The emulsion grains thus formed then serve as hosts for further growth. Once a host grain population has been prepared, an increased concentration of iodide is introduced into the emulsion to form the region of the grains containing maximum iodide concentration. The source of iodide ion can be silver iodide grains or any iodide-releasing agent, but it is typically disclosed that iodide is preferably introduced alone as an aqueous solution of an alkali metal iodide salt. This is followed by double-jet introduction of silver nitrate and alkali metal chloride solutions at conventional molar addition rates, constant or ramped, till the exterior portion is grown to the desired size.
Many known imaging systems require that a hard copy be provided from an image which is in digital form. A typical example of such a system is electronic printing of photographic images which involves control of individual pixel exposure. Such a system provides greater flexibility and the opportunity for improved print quality in comparison to optical methods of photographic printing. In a typical electronic printing method, an original image is first scanned to create a digital representation of the original scene. The data obtained is usually electronically enhanced to achieve desired effects such as increased image sharpness, reduced graininess and color correction. The exposure data is then provided to an electronic printer which reconstructs the data into a photographic print by means of small discrete elements (pixels) that together constitute an image. In a conventional electronic printing method, the recording element is scanned by one or more high energy beams to provide a short duration exposure in a pixel-by-pixel mode using a suitable source such as a cathode ray tube (CRT), light emitting diode (LED) or laser. Such methods are described in the patent literature, including, for example, Hioki U.S. Pat. No. 5,126,235; European Patent Application 479 167 A1 and European Patent Application 502 508 A1. Also, many of the basic principles of electronic printing are provided in Hunt, The Reproduction of Colour, Fourth Edition, pages 306-307, (1987).
Reciprocity characteristics, usually referred to as reciprocity failure, are measured in terms of departures from the law of photographic reciprocity. The exposure (E) of a photographic element is the product of the intensity (I) of exposure multiplied by its duration (time): EQU E=I.times.time
According to the photographic law of reciprocity, a photographic element should produce the same image with the same exposure, even though exposure intensity and time are varied. For example, an exposure for 1/100.sup.th of a second at a selected intensity should produce exactly the same result as an exposure of 10.sup.-5 second at an intensity that is increased by a factor of 10.sup.3. When photographic performance is noted to diverge from the reciprocity law, this is known as reciprocity failure.
A very typical observation in examining high chloride emulsions for photographic print applications is that speed declines at equal exposures as the intensity of exposure increases. For equal exposures, a speed difference at the exposure time of 10.sup.-5 second or less, as compared to an exposure time of 1/100.sup.th of a second is commonly referred to in the art as high intensity reciprocity failure (HIRF). Likewise, the exposure times greater than 1/100.sup.th second are often referred to as "long time" exposure, whereas those shorter that that as "short time" exposures.
In order to increase the output of digital printing devices, such as CRT, LED, or laser-based printers it is highly desirable to increase speed of high chloride silver halide emulsions when exposed at very short times even further. In the art of silver chloride-based color paper preparation it is the blue color record that has the greatest need for speed.
The use of dopants in silver halide grains to modify photographic performance is generally illustrated by Research Disclosure, Item 38957, cited above, I. Emulsion grains and their preparation, D. Grain modifying conditions and adjustments, paragraphs (3)-(5). Photographic performance attributes known to be affected by dopants include sensitivity, reciprocity failure, and contrast.
Using empirical techniques the art has over the years identified many dopants capable of increasing photographic speed. Keevert et al U.S. Pat. No. 4,945,035, e.g., was the first to teach the incorporation of a hexacoordination complex containing a transition metal and cyano ligands as a dopant in high chloride grains to provide increased sensitivity. Careful scientific investigations have revealed Group VIII hexahalo coordination complexes to create electron traps, as illustrated R. S. Eachus, R. E. Graves and M. T. Olm J. Chem. Phys., Vol. 69, pp. 4580-7 (1978) and Physica Status Solidi A, Vol. 57, 429-37 (1980) and R. S. Eachus and M. T. Olm Annu. Rep. Prog. Chem. Sect. C. Phys. Chem., Vol. 83, 3, pp. 3-48 (1986). Doping with iridium hexachloride complexes, e.g., is commonly performed to reduce reciprocity law failure in silver halide emulsions. The use of iridium dopants containing at least one organic ligand has also been proposed. Specific iridium dopants include those illustrated in high chloride emulsions by Bell U.S. Pat. Nos. 5,474,888, 5,470,771 and 5,500,335 and McIntyre et al U.S. Pat. No. 5,597,686; those disclosed in Olm et al U.S. Pat. Nos. 5,360,712 and 5,457,021; Kuromoto et al U.S. Pat. No. 5,462,849; Mydlarz et al U.S. Pat. No. 5,783,373 and U.S. Pat. No. 5,783,378; Hahm et al U.S. Pat. No. 5,962,210. Specific combinations of iridium and other metal dopants may additionally be found in U.S. Pat. Nos. 4,828,962, 5,153,110, 5,219,722, 5,227,286, and 5,229,263, copending, commonly assigned U.S. Ser. No. 09/250,200 of Mydlarz et al., filed Feb. 16, 1999, and European Patent Applications EP 0 244 184, EP 0 405 938, EP 0 476 602, EP 0 488 601, EP 0 488 737,EP0 513 748,and EP0 514 675.
In general, the prior art dopant teachings typically use silver chloride as a host medium and merely disclose that photographic emulsions may contain a variety of halides, including silver iodochloride. They do not, in general, even reference silver iodochloride emulsions in accordance with this invention (those with local regions of high iodide concentration resulting from the rapid addition of iodide ion at some point during the second half of grain formation).
To further improve the photographic performance of high chloride emulsions bromide is often introduced during the process of chemical sensitization or ripening. Hasebe et al U.S. Pat. No. 4,865,962 (a) provides regular grains that are at least 50 (preferably at least 90) mole percent chloride, (b) adsorbs an organic compound to the grain surfaces and (c) introduces bromide, thereby achieving halide conversion (bromide ion displacement of chloride) at selected grain surface sites. Asami EPO 0 295439 discloses the addition of bromide to achieve halide conversion at the surface of silver bromochloride grains that have, prior to halide conversion, a layered structure with the surface portions of the grains having a high chloride concentration. The grains are preferably monodisperse. Suzumoto et al U.S. Pat. No. 5,252,454 discloses silver bromochloride emulsions in which the chloride content is 95 (preferably 97) mole percent or more. The grains contain a localized phase having a bromide concentration of at least 20 mole percent preferably formed epitaxially at the surface of the grains. The grains are preferably monodisperse. Oshima et al U.S. Pat. No. 5,252,456 discloses silver bromochloride emulsions in which the chloride content is at least 80 (preferably equal or greater that95) mole percent chloride, with a bromide rich phase containing at least 10 mole percent bromide formed at the surface of the grains by blending a fine grain emulsion with a larger host (preferably cubic or tetradecahedral) grain emulsion and Ostwald ripening. An iridium coordination complex containing at least two cyano ligands is employed to increase speed and reduce reciprocity failure.
Edwards et al U.S. Pat. No. 5,792,601 discloses a radiation sensitive emulsion containing iridium doped composite silver halide grains comprised of (a) host portions having an average aspect ratio of less than 1.3 and consisting essentially of monodisperse silver iodochloride grains containing from 0.05 to 3 mole percent iodide, based on total silver forming the host portions, with maximum iodide concentrations located nearer the surface of the host portions than their center and (b) epitaxially deposited portions containing an iridium dopant and silver bromide accounting for from 0.1 to 5 mole percent of total silver forming the composite grain. The source of iodide used to prepare the host iodochloride portions in all the inventive examples is potassium iodide solution.
It has become increasing clear that with the continuing development of a variety of high intensity digital printing devices that photographic print materials with performance invariant to exposure time is increasingly important. When exposure times are reduced below one second to very short intervals (e.g., 10.sup.-5 second or less), higher exposure intensities must be employed to compensate for the reduced exposure times. High intensity reciprocity failure (hereinafter also referred to as HIRF) occurs when photographic performance is noted to depart from the reciprocity law when such shorter exposure times are employed. Print materials which traditionally suffer speed or contrast losses at short exposure times (high intensity exposures) will fail to reproduce detail with high resolution. Text will appear blurred. Through-put of digital print devices will suffer as well. Accordingly, print materials with reduced HIRF are desired in order to produce excellent photographic prints in a wide variety of digital printers.
In addition to reducing HIRF, it is also desirable to reduce low intensity reciprocity failure (LIRF) in photographic elements. Print materials with reduced LIRF, e.g., will allow enlargements of photographs to be made by conventional optical printing techniques with a more faithful matching of image tone and color.